Receiver for a. m. speech channel having means to eliminate effects of superimposed frequency shift keying



I Ocf- 1, 1957 D. v. SINNINGER 2 Sheets-Sheet l Filed Dec. 31, 1953 Oct.1, 1957 n. v. slNNlNGER 2,808 ANS RECEIVER FOR A. M. SPEECH CHANNELHAVING ME TO ELIMINAT E EFFECTS OF SUPERIMPOSED FREQUENCY SHIFT KEYING 2Sheets-Sheet 2 Filed Dec. 31. 1953 INVENTOR.

United States Patent O RECEIVER FOR A. M. SPEECH CHANNEL HAVING MEANS T0ELLVIIYATE EFFECTS OF SUPER- IMPOSED FREQUENCY SHIFT KEYING Dwight V.Sinninger, Oak Park, Ill., assigner, by mcsne assignments, to HuppCorporation, Cleveland, Ollio, a corporation of Virginia ApplicationDecember 31, 1953, Serial No. 401,487

12 Claims. (Cl. Z50-20) It is a principal object of this invention toprovide an improved radio receiver capable of reception of twoindependent but coincident intelligences without interferencetherebetween.

Several systems have heretofore been proposed by which a common carriermay be utilized to transmit and receive an amplitude modulated (AM)intelligence and coincidently carry a teletype message by a frequencyshift (FS) of the carrier. In simple systems of transmission andreception, this combination of functions is found completely compatible,and little or no interference between the two modes of intelligencetransmission is produced. in such simple systems a conventional receiveris employed having appropriate stages for radio frequency amplification,frequency conversion, intermediate frequency amplification, detectionand audio amplification. In addition thereto, the signal generated bythe intermediate frequency, or I. F., amplifiers is applied to a limiterand frequency discriminator which detects a shift in the carrierfrequency and generates a signal in accordance with such shift wherebythe output may be utilized to operate a teletypewriter. The audio outputis substantially unaffected by the frequency shifts as all of the stagesare broadly tuned to accommodate sufficient band width to pass thecarrier and sidebands over a range in excess of 200 cycles, the maximumfrequency shift generally employed in teletype transmission which isaccompanying amplitude modulation signals.

However, to receive an amplitude modulated signal with optimum fidelityand minimum distortion and interference it is desirable to employ areceiving apparatus which is capable of altering the signal as receivedbefore passing the signal to the final detecting or demodulating portionof the receiver. To avoid distortion in the detected signal it isnecessary that the carrier have, at all times, an amplitude greater thanthat of the sidebands which represent the intelligence being conveyed.To insure this mode of operation, receivers are provided which isolatethe carrier from the sidebands, customarily by the use of filters,amplify the carrier to a predetermined level, and rcinsert the amplifiedcarrier into the basic signal having the sidebands whereby thedistortion which would otherwise result from overmodulation iseliminated.

A second common source of distortion in received radio signals is thatwhich results from relative phase shift between the two sidebands of agiven signal. To avoid the distortion which would be produced by such aphase shift, receivers are provided which remove one of the sidebandsfrom the incoming signal and utilize a single sideband in the detector.A single sideband carries all of the intelligence which is transmittedfrom a given transmitter. In removing one of the sidebands from anincoming signal, diminution of the carrier will generally result becauseof the inherent characteristics of the band pass filters employed. Tocompensate for this attenuation and to gain other desired actions it iscustomary to provide an amplified or exalted carrier in single sidebandreception also.

When utilizing these improved modes of reception of amplitude modulatedsignals it has been found that any rapid changes in the frequency of theincoming wave will produce a tremolo or other objectionable interferencein the resulting audio signal as a result of the phase shift of theexalted carrier which is not present in the original carrier andsidebands. The phase shift in the exalted carrier results primarily fromthe passage of said carrier through a narrow band pass filter circuit.

Therefore it is a further object of this invention to provide improvedradio receiving apparatus which is adapted to receive a signal carryingintelligences thereon produced both by amplitude and frequency variationwherein each variable is free of interference or distortion which mightbe produced by the other.

It is another object of this invention to provide improved radioreceiving apparatus adapted for reception of a carrier having both anamplitude and frequency varying characteristic in which no tremolo isproduced in the audio output after demodulation of the AM signal as aresult of a frequency shift of the incoming carrier.

l t is still another object of this invention to provide radio receivingapparatus adapted for single sideband reception and frequency shiftteletype reception wherein the amplitude modulated signal will notevidence any distortion or interference from the frequency shift.

lt is still another object of this invention to provide improved radioreceiving apparatus utilizing sideband selection and exalted carriermeans for improving the reception of amplitude modulated signals, saidreceiver also being adapted to receive frequency shift intelligencewithout producing distortion in the two intelligence channels.

lt is a further object of this invention to provide improved radioreceiving apparatus utilizing an exalted carrier circuit wherein thephase of the exalted carrier is automatically adjusted to maintainsubstantial phasic agreement between the incoming signal and the exaltedcarrier whereby distortion and interference are minimized.

It is still another object of this invention to provide a closed cyclecontrol system in which the phase of an exalted carrier in a radioreceiver is automatically maintained in substantial agreement with thephase of an incoming signal.

It is a further and important object of this invention to provide anautomatic frequency control system which may be utilized in a receiveradapted for reception of signals having amplitude modulation, frequencyshift intelligence or both impressed thereon.

It is still another object of this invention to provide an improvedsystem for high fidelity audio reception and coincident frequency shiftteletype reception which may be readily installed in existing equipment,which is not subject to failure, and which does not require extensivemaintenance or adjustment.

It is a further object of this invention to provide an improved systemfor rendering a diversity receiver adapted for single sideband andexalted carrier reception compatible with frequency shift teletypereception.

Further and additional objects of this invention will become manifestfrom a consideration of this description, the accompanying drawings, andthe appended claims.

In one form of this invention a radio receiver is provided which isadapted for operation in multiple modes whereby various known sources ofinterference and distortion may be eliminated. The receiver furtherprovides for the reception of teletype intelligence which is transmittedby a narrow frequency shift of the amplitude modulated signal. A signalis received from the transmitting station which contains audiointelligence, amplitude modulated onto a carrier in the customaryfashion. In addition thereto teletype intelligence has been irnposedupon the signal by shifting both the carrier and the sidebands of saidsignal to a new predetermined frequency within a limited range overwhich the normal AM receiving apparatus will function withoutdistortion. Four independent modes of operation are provided in thisreceiver, namely a normal mode, in which the incoming signal is merelyamplified and detected; an exalted carrier mode in which the carrier isisolated from the sidebands and independently amplified, after which itis reinserted into the original signal including sidebands, thusavoiding any danger of distortion from overmodulation; an upper sidebandexalted carrier mode in which the carrier is segregated andindependently amplified and the lower sideband is eliminated, wherebyinterference which might otherwise result from overmodulation orsideband phase shift is eliminated; and lowerside band exalted carriermode reception in which the upper sideband is eliminated and the carrierindependently amplified for the same purpose as that just described. Asthe carrier is isolated from the associated sidebands by passage througha sharply tuned band pass filter, the shifting of the carrier intransmitting teletype intelligence causes a substantial phase shift atthe output of the carrier filter. This phase shift will produceobjectionable thumping or tremolo when the carrier is recombined withthe sidebands. To eliminate this interference the carrier is passed fromthe band pass filter to a dynamic phase shifting circuit whichautomatically adjusts the phase to bear a fixed relationship to that ofthe original signal with sidebands. The dynamic phase shifting circuitis energized from an automatic phase controlling discriminator whichcontinuously compares the phase of the incoming signal with that of theexalted carrier and generates a voltage which corresponds directly tothe magnitude of phase deviation.

The output of the carrier filter is also utilized to energize aquadrature phase shifting circuit, the output of which is applied to abalanced discriminator which receives a second voltage corresponding tothe carrier before passage through the band pass filter. Thisdiscriminator produces a voltage correspending to the phase shift of thecarrier in passing through the carrier filter which in turn bears apredetermined relationship to the frequency shift of the incomingsignal. The output of this discriminator serves two purposes: It isapplied to a -L variable reactance circuit after passage through a timeconstant network, the output of said variable reactance circuitdetermining the frequency which is maintained in the intermediatefrequency stages of the receiver prior to the circuits herein described.The discriminator output is also applied through a low pass filter andclipper, clamp, and D. C. restorer to a D. C. amplifier which directlyenergizes a teletype printer to reproduce the teletype intelligencetransmitted by the frequency shift of the incoming signal.

For a more complete understanding of this invention reference will nowbe made to the accompanying drawings, wherein:

Fig. 1 is a block diagram of certain circuits of a radio receiver whichillustrates one embodiment of this invention;

Fig. 2 is a circuit diagram illustrating the contents of certain of theblocks illustrated in Fig. 1; and

Fig. 3 is a graphical illustration of the phase and magnitude of theoutput of the carrier band pass filter as compared to the input thereto.

Referring now to the drawings, and more particularly to Fig. 1, thesideband selector, carrier exaltation and detector portions of a fourmode superheterodyne receiver are illustrated. The portions of thereceiver i1- iif) lustrated in Fig. 1 may be one of two or moreidentical systems employed in conjunction with identical receivers asutilized in a diversity receiving system wherein the various outputs ofthe individual receivers are automatically combined in a control unit toproduce the optimum quality in the resulting audio signal with minimumfading and distortion. This automatic control in combining the signalsmay readily be accomplished by generating an automatic gain control D.C. voltage which is fed back to all of the independent receivers but themagnitude of which bears a fixed relationship to the sum of all of thesignal strengths, or to the stronger signal. However, for the purposesof this invention a single detector circuit will be described as each ofthe multiple receivers functions in an identical manner.

ln Fig. l the received signal is applied to the mixer from previousstages of amplification as indicated by arrow 12. The signal is thereinheterodyned with a signal from conversion oscillator 42 in thewell-understood manner to produce an intermediate frequency which in oneparticular embodiment of this invention is 137 kilocycles. Thisintermediate frequency is applied to the l. F. amplifier 14, where it isamplified to a predetermined voltage level. The output of I. F.amplifier 14 is applied to limiter 16, automatic gain control voltagegenerator 18, diode detector 20, upper sideband filter 22, lowersideband filter 24, and exalted carrier balanced demodulator 26. Theoutput of I. F. amplifier 14 represents the normal signal produced by acustomary superheterodyne receiver, and is applied to diode detector fornormal mode reception, utilized primarily for tuning and adjustment. Theoutput of automatic gain control (AGC) voltage generator 18 is a D. C.signal which is combined. with other AGC signals from other receivers ofthe system and fed back to the I. F. amplifier 14 and the variousprevious stages of the receivers to produce a substantially constantlevel output from the receivers in the well-known manner.

The limiter 16 may be of any well-known type but is preferably anoverloaded amplifier which will produce a substantially constant outputirrespective of the magni tude of the signal applied at its input. Thusthe amplitude modulated intelligence present in the incoming signal issubstantially removed by limiter 16 but any frequency shift intelligencewill be passed therethrough. The output of limiter 16 is applied tocarrier filter 28, automatic frequency control and frequency shiftdetector 30, and automatic phase control discriminator 32. The output ofthe carrier filter 2S, which is sharply tuned, will be a substantiallypure carrier frequency having no interference, noise, or sidebandsassociated therewith. The output of filter 28 is applied to dynamicphase shifter 34 and quadrature phase shifter 36. The output ofquadrature phase shifter 36 is applied to AFC and frequency shift (FS)detector 30. Detector is a balanced demodulator serving as adiscriminator with the quadrature voltage supplied from the carrierfilter 28, as will be explained in detail with respect to Fig. 2,whereby small frequency shifts in the incoming signal will produce alarge output signal which may be utilized to generate both an automaticfrequency control signal and a signal for operation of a teletypeprinter. The output of discriminator 30 is applied to an AFC timeconstant network 38 which may be one of a plurality of well-knowncircuits, the simplest of which is a parallel condenser and seriesresistor whereby the full potential will not appear at the output ofnetwork 38 for a predetermined time, depending upon the time constant ofthe network, follow ing the application of a potential at the inputthereof. The purpose of this network is to apply the output voltage fromthe discriminator to the variable reactance circuit 40 only forfrequency changes which persist over a relatively long period of time.Thus by proper selection of the components of network 38, the variablere actance 40 will not be affected by the reception of individualteletype bands but will adjust the steadystate frequency of the outputsignal from mixer which is applied to the I. F. amplifier 14 by alteringthe frequency of the conversion oscillator 42 in accordance with thesustained averaged output of discriminator 30.

The output of discriminator is also applied to low pass filter 54, whicheliminates the carrier and any spurious noise or other interference andpasses only the D. C. impulses representing teletype bands. The outputof filter 54 is applied to clipper, clamp, and D. C. restorer 56,wherein the signal is amplified and its D. C. level adjusted to producean output comprising a series of timed D. C. impulses or bandscorresponding to the teletype intelligence received from thetransmitter. The output of the D. C. restorer is applied to D. C.amplifier 58, v

which in turn energizcs any conventional type of teletypewriter.

The isolated carrier available at the output of filter 28 is applied todynamic phase shifter 34, wherein the signal is amplified and its phaseadjusted so that the output bears a predetermined phasic relationship tothe signal applied from the l. F. amplifier 14 to the sidcband filters22 and 24 and the balanced demodulator 26. This reconditioned carrier isapplied to a manually adjustable phase shifter 44, the output of whichis applied to balanced demodulator 26, upper sideband balanceddernodulator 46, and lower sideband balanced demodulator 48. Thereconditioned carrier is utilized along with the incoming signal indemodulator 26, which may be any one of several well-known circuits suchas a product demoduiator. to produce un audio output indicated by arrowwhich may be amplified by customary audio means for utilization asdesired, generally for rebroadcast. The reconditioned carrier is appliedto the sideband demodulaters 46 and 48 where it is compared with theoutput of sideband filters 22 and 24 to produce an audio signal at theoutputs of said demodulators in the same manner as in the exaltedcarrier demodulator 26.

The output of dynamic phase shifter 34 provides the reconditionedcarrier for subsequent dcrnodulation as described but is also applied toa quadrature phase shifting circuit 52, the output of which is appliedto the automatic phase control discriminator 32. Discriminator 32compares the instantaneous phase of the isolated carrier with that ofthe output of limiter 16 and produces D. C. error voltages which areapplied to the dynamic phase shifter 34 to alter the phase of theisolated carrier in a manner to be described with respect to Fig. 2.

The elements of the more important blocks of Fig. l

are shown in detail in Fig. 2. Therein the incoming signal is applied tothe limiter 16 from the i. F. amplifier through conductor 60. Asdescribed above, the limiter substantially removes the amplitudemodulation from the incoming signal, after which the signal is appliedto the phase controlling discriminator 32, carrier filter 28, and AFCand frequency shift detector 30. The output of carrier filter 23 isapplied to the primary winding of a balanced transformer 62 in thedynamic phase shifter 34. The secondary winding of transformer 62 has acenter tap 64 which is connected to ground and terminals 66 and 68 whichare energized by the primary to produce equal voltages in 180 phaserelation. The Voltage ap rearing between terminal; rnd of thetransoriner 62 is applied to two networks 76 and 72, each comprising aresistance and capacitance in series, said networks being connected inparallel with the elements inverted. A conductor extends from the centerof each of the networks '70 and 72 and is capacitively coupled throughrespective coupling eondensers 74 and 76 to the control grids ofappropriately connected pentodes 78 and 80. The networks and 72 are soselected and oriented that the voltage applied to the grid of pentode 78will normally lag the applied voltage to the network by 60 while thevoltage applied to the control grid of pentode 80 will lead the voltageapplied to the network by 60. The plate crcuits of pentodes 78 and 80are connected in parallel to a common output comprising the primary ofan output transformer 82 and a condenser 84 in parallel therewith. Asthe pentodes are energized by signals which are normally 120 out olphase and their plate circuits connected in parallel, the signalappearing across the secondary of transformer 82 will be a vectorresultant of the two independent pentode outputs and will have a phaseunder normal conditions intermediate the two output signals ofapproximately 180 with respect to the voltage initially implied to thenetworks 70 and 72 after undergoing full phase reversals in the pentodetubes. A. D. C. bias voltage may be applied to the control grid ofpentode 78 from the phase control discriminator 32 through resistor 86.ln a like manner, the phase control discriminator 32 may provide a D. C.bias voltage through resistor 88 to the control grid of. pentode 80. Ifa negative bias voltage is applied to either but not both of thesepentodes the conduction therethrough will be diminished, as is believedwell understood, which will produce an effective phase shift in theoutput voltage appearing across transformer 82.

The output of transformer 82 is applied to the primary of a balancedtransformer 90 of the manual phase adjuster 44. The secondary oftransformer 90 has a center tap which is connected to ground and a pairof terminais, one of which is connected to a variable resistor 92 andthe other of which is connected to condenser 94. A conn-non connectionbetween resistor 92 and condenser 94 is also connected to the controlgrid of tube 96, the cathode output of which is applied to the balanceddemodulators 26, 46, and 48 of Fig. 1. The tube 96 has a cathoderesistor 98 and constitutes a cathode follower whereby the circuitexhibits great stability, current arnplifying properties, and improvedmatching characteristics.

The adjustment of resistor 92 will determine the phase ot' the signalapplied to the demodulators and may be critically adjusted to remove anysteady or static errors in phase.

As described previously the output of the manual phase adjuster 44,which is a phase corrected exalted carrier signal, is applied to each ofthe balanced demodulators 26, 46, and 48. The basic signal is applieddirectly to demodulator 26 from the I. F. amplifier 14 and is passedthrough upper sideband lter 22 to demodulator 46 and through lowersideband filter 24 to demodulator 48. The output from each of thedemodulators will be an audio signal corresponding to the AMintelligence, and by a manually selective arrangement the undistortedaudio output ol one of the demodulators will be utilized.

The output of transformer 82 is also applied Vto the primary of atransformer 10i? tof quadrature phase shifting circuit 52. The secondaryof transformer 100 has a grounded center tap and a network comprisingadjustable resistor 102 and condenser 104 serially connected across itsterminals. The resistor 102 is adjusted whereby the signal applied tothe control grid of cathode follower 106 will bear a 90 phaserelationship to the applied voltage. The output of cathode follower 106is taken across cathode resistor 108 and applied through couplingcondenser 110 to the serially connected transform-ers 112 and 114 of a.pair of associated balanced demodulators in the phase controldiscriminator 32.

The output of limiter 16 is also applied to the tbalsneed demodulatorsthrough center tapped transformers 116 and 118. The balanceddemodulators are connected in a fashion whereby the center taps oftransformers 116 and 118 are connected to the secondaries oftransformers 112 and 114 respectively, the other windings of which `areconnected to the common terminals of catho-de resistors 120 and 122 andcathode resistors 134 and 136 of two pairs of diode rectifiers 124, 126,and 125, 127. A condenser 128 is connected in parallel with the cathoderesistor 120 to sustain a D. C. voltage thereacross, and a similarfilter condenser 130 is provided in parallel with cathode resistor 122.A condenser 132 is provided in parallel with the secondary of centertapped transformer 116 and is selected to resonate at substantially thecarrier frequency and a condenser 133 is connected in parallel with thesecondary of transformer 118 for the same purpose.

Though the manner in which a single balanced demodulator functions isbelieved clear, a general description of the operation follows. If thevoltage applied to transformer 112 bears precisely a 90 phaserelationship to the voltage applied to transformer 116, thc voltagesappearing across resistors 120 and 122 will `be of equal and oppositemagnitude, and consequently the net voltage at the cathode of dio-de 124with respect to ground will be zero. If, however, there is a phase shiftproduced in either the incoming signal or the isolated carrier signalwhich is not produced in the other signal, the voltage applied totransformer 112 will no longer bear a 90 phase relationship with respectto the voltage applied to transformer 116. ln that event the voltageappearing across resistor 128 or 122 will exceed the voltage appearingacross the other of said resistors by a `magnitude corresponding to thephase shift. This is believed manifest in that a phase shift in thevoltage applied to transformer 112 will cause thc signal applied theretoto add to the voltage in one-half of the secondary of transformer 116,while the same phase shift in the voltage applied to the transformer 112will produce a vector subtraction with respect to the voltage in theother half of the transformer 116.

The second balanced demodulator, comprising the other half of the dualphase control discriminator 32 of this invention, `operates in a manneridentical with that just described, whereby the voltages appearingacross resistors 134 and 136 will be equal and opposite when the phaseposite polarity. These equal and opposite potentials are appliedrespectively to the control grid of the pentodes 78 and 80 in thedynamic phase shifting circuit 34 to pro duce a bias on one tube toeffect greater conduction and on the other to effect decreasedconduction to produce a phase shift as above described in the output ofthe t circuit 34 which tends to bring said output into phasic agreementwith the output of limiter 16. Thus the dynamic phase shifting circuit34, quadrature phase shifting circuit 52, and dynamic phase controldiscriminator 32 comprise a closed cycle control wherein thediscriminator compares the phase of the input to the filter 28 with theoutput of phase shifter 52 and generates an error signal as determinedby that comparison. The error signal applied to the dynamic phaseshifting circuit 34 will reduce any phase discrepancies between the twosignals toward zero.

A typical response curve for a band pass filter such as carrier lter 28is illustrated by trace 138 of Fig. 3. The curve is symmetrical about acenter axis 140 which represents the carrier frequency F0 which ispassed through the filter substantially unattenuated and on either sideof which rapid and severe attenuation occurs. The lower curve 142,representing the phase shift produced in the carrier filter 28, clearlyillustrates the need for the phase correcting circuits herein describedwhen a system is employed utilizing both amplitude modulation andfrequency shift modulation.

The output of limiter 16 is also applied to AFC and frequency shiftdetector 30 as described with respect to Fig. l. Detector 30 is abalanced demodulator in which bil the output of limiter 16 is applied tothe primary winding of a center tapped transformer 144. The output ofcarrier filter 28 is applied to center tapped transformer 146 ofquadrature phase shifting circuit 36 which is identical to thequadrature phase shifting circuit 52 above described. A networkconnected seriatim across the center tapped secondary of transformer 146comprises a resistor 148 and a condenser 150 having substantially equalimpedance, whereby the voltage applied to the grid of cathode follower152 will bear a 90 phase relationship to the input voltage. The outputof this circuit is taken across cathode resistor 154 and applied throughcoupling condenser 156 to transformer 158 of the balanced demodulator30. The secondary of transformer 158 is connected ybetween the centertap of the transformer 144 and the common connection of cathoderesistors 160 and 162. This balanced -demodualtor operates in a manneridentical to the first half of the phase control diseriminator 32 abovedescribed in detail and will produce a D. C. output voltage at conductor164 which corresponds in magnitude and sense to the phasic relationshipof the voltages applied to transformers 144 and 1518. Thus a frequencyshift of the incoming signal will produce la phase shift in carrierfilter 23 which consequently produces a phase shift in the voltageapplied to transformer 158, and a D. C. output at conductor 164. Becauseof the sharp tuning characteristic of carrier filter 28 the phase shiftproduced for small frequency shifts will be substantial and thusproduces a large D. C. output `from discriminator 30. This D. C. output,as is believed clear `from the description of Fig. l, is utilized bothfor `automatic frequency control and teletypewriter operation. Thesignal is applied through AFC time constant net- Work 38 to variablereactance 40, where, as `an averaged D. C. potential, it determines thefrequency of conversion oscillator 42, which in turn adjusts thesteady-state intermediate frequcncy to a predetermined valuesubstantially corresponding to the center frequency Fo of the carrierfilter 28. The signal is also applied through low pass filter 54,clipper, clamp, and D. C. restorer 56, and D. C. amplifier 58 to anyconventional teletype printing equipment. The low pass filter functionsto eliminate noise and other undesirable signals from the teletypesignals, while the clipper removes the peaks from the incoming impulsesproducing a substantially square wave shape. The clamp establishes aknown zero datum for the square wave signal and the D. C. restorerestablishes this zero datum at any desired level, customarily with themaximum negative excursions corresponding to zero voltage. Thus whenapplied to a printer, the printer is actuated only by the positiveimpulses which it receives. Various circuitry for accomplishing thesefunctions is well known in the art.

Generally the frequency shift which may be tolerated in normal modereception of amplitude modulated signals without noticeable tremolo orthumping in the received audio is within a range of 10-200 cycles. Ithas been `found that when utilizing .single sideband reception and anexalted carrier, frequency shifts of the order of 20 cycles produceobjectionable distortion and interference in the output audio. This isnot a result of the frequency change, as both carrier and sidebands arealtered in `frequency shift teletype transmission, but interferenceresults instead from the phase difference between the sidebands and theisolated carrier which has been conditioned by passage through a sharplytuned filter and amplifier. By this invention the customary diticultiesexhibited by the superior exalted carrier type of receiver in receivinga signal having both amplitude modulation and frequency shiftintelligence are completely overcome. This is accomplished not byeliminating the phase shift of the isolated carrier but by producing acompensating phase shift through the closed cycle control whichcomprises the automatic phase control discriminator 32.and dynamic phaseshifter 34.

While one particular embodiment has herein been described utilizingamplitude modulation and frequency shift teletype transmission, it isbelieved manifest that many embodiments and modifications will Vbecomeobvious to one skilled in the art. For example, while in this embodimentupper and lower sideband filters are employed to isolate a particularsideband for use in a particular mode of reception, the same result maybe accomplished by supplementing filter selectors with wide band audiophase shifting or by using the latter technique exclusively.Furthermore, while the AFC time constant network circuit in thisembodiment is stated to comprise a simple resistance capacitancenetwork, other large time constant techniques may be employed, includingthe utilization of the Miller effect in a vacuum tube for capacitancemultiplication.

While ia particular embodiment of the invention has been shown, it willbe understood, of course, that the invention is not limited theretosince many modifications may be made, and it is, therefore, contemplatedby the appended claims to cover any such modifications as fall withinthe true spirit and scope of the invention.

The invention having thus Ibeen described, what is claimed and desiredto be secured by Letters Patent is:

l. In apparatus for receiving radio signals having a carrier andassociated sidebands, means for separating such carrier from suchsidebands, means `for altering su-ch carrier in a predetermined manner,means for continuously adjusting the phase of such altered carrier inaccordance with the phasic relationship between the carrier portion ofsaid signals and the altered carrier to bear a predetermined phasicrelationship to such sidebands, and means for recombining such alteredcarrier and such side bands.

2. In apparatus for receiving radio signals having a carrier andsidebands, means for separating such carrier from such sidebands, meansfor amplifying such carrier to a predetermined level, means forcontinuously adjusting the phase of such amplified carrier in accordancewith the phasic relationship between the carrier portion of said signalsand the altered carrier to bear a predetermined phasic relationship tosuch sidebands, and means for recombining such amplified carrier andsuch sidebands.

3. ln apparatus for receiving radio signals having Va carrier andassociated sidebands, means Vfor separating such carrier from suchsidebands, means for altering such carrier in a predetermined manner,closed cycle control means for adjusting the phase of such alteredcarrier in accordance with the phasic relationship between the carrierportion of said signals and the altered carrier to bear a predeterminedphasic relationship to such sidebands, and means for recombining suchadjusted and altered carrier and such sidebands.

4. In apparatus for receiving radio signals having a carrier andassociated sidebands, narrow band filter means for separating suchcarrier from such sidebands, means for amplifying said carrier to apredetermined level, dynamic `phase adjusting means for adjusting thephase of such separated carrier, and means for comparing the phasicrelationship of said carrier at the input to said filter and at theoutput of said dynamic phase adjusting means to generate a controlsignal corresponding to said relationship, said signal being applied tosaid control phase adjusting means to alter the phase of the outputthereof to correspond to the phase of the input to said filter.

5. In apparatus for receiving radi-o signals having a carrier andassociated sidebands for conveying intelligence as a function of boththe amplitude and frequency' of the signals, amplitude limiting meansenergized from said signals, narrow `band lter means energized from saidlimiting means to separate such carrier `from such sidebands, dynamicphase adjusting means for determining the phase of the separated carrierrelative to the carrier portions of said signals, and means forcomparing the phasic relationship of said carrier at the input to saidCil filter and at the output of said phase adjusting means to generate acontrol signal corresponding to said relationship, said control signalbeing applied to said phase adjusting means to alter the phase of theoutput thereof to correspond to the phase of the input to said lter.

6. in apparatus for receiving radio signals having a carrier andassociated sidebands for conveying intelligence as a function of boththe amplitude and frequency of such signals, amplitude limiting meansenergized from such signals, narrow band filter means energized fromsaid limiting means to separate such carrier from such sidebands,dynamic phase adjusting means for determining the phase of the separatedcarrier relative to the carrier portions of said signals, means forcomparing tite phasic relationship of said carrier at the input to saidfilter and at the output of said phase adjusting means to generate acontrol signal corresponding to said relationship, said control signalbeing applied to said phase adjusting means to alter the phase of theoutput thereof to correspond to the phase of the input to said filter,and means to recombine said adjusted carrier with the original radiosignal.

7. In apparatus for receiving radio signals having a carrier andassociated upper and lower sidebands for conveying intelligence as afunction of both the amplitude and frequency of such signal, narrow bandfilter means energized by said radio signal to separate the carriertherefrom, dynamic phase adjusting means for determining the phase ofthe separated carrier relative to the carrier portions of said signals,means for Comparing the phasic relationship of said carrier at the inputto said filter and at the output of said phase adjusting means togenerate a control signal corresponding to said relationship, saidcontrol signal being applied to said phase adjusting means to alter thephase of the output thereof to bear a predetermined phasic relationshipto the input to said filter, means to recombine said adjusted carrierwith the original radio signal, and detector means energized from theinput to said filter and the output therefrom to generate a directcurrent voltage corresponding to the phase relationship thereof, saidvoltage indicating the inteligence imposed on said radio signal :is afunction of the frequency thereof.

8. ln apparatus for receiving radio signals having n carrier andassociated upper and lower sidebands for conveying intelligence as afunction of both the amplitude and frequency of such signal, narrow bandfilter means energized by said radio signal to separate the carriertherefrom, dynamic phase adjusting means for determining the phase ofthe separated carrier relative to the carrier portions of said signals,means for comparing the phasic relationship of said carrier at the inputto said filter and at the output of said phase adjusting means togenerate a control signal corresponding to said relationship, saidcontrol signal being applied to said phase adjusting means to alter thephase of the output thereof to bear a predetermined phasic relationshipto the inout of to said filter, means to recombine said adjustedtrarrier with the original radio signal, quadrature phase shifting meansenergized from the output of said filter means` discrimioator meansenergized from the input signal to said filter means and the output fromsaid quit-trattare phase shifter whereby frequency shifts of theiucomim.` signal produce a corresponding direct current voltaae in saiddiscriminator, and time constant means energized from said discriminatormeans, the output of said time constant means corresponding tofrequency' changes of such radio signal of substanitally longer timeduration than the intelligence conveying frequency variations.

9. ln apparatus for receiving radio signals having a carrier andassociated upper and lower sidebands for conveying intelligence as afunction of both the amplitude and frequency of such signal, narrow bandfilter means energized by said radio signal to separate the carriertherefrom, dynamic phase adjusting means for determining the phase ofthe separated carrier, means for comparing the phasic relationship ofsaid carrier at the input to said litter and at the output of said phaseadjusting means to generate a control signal corresponding to saidrelationship, said control signal being applied to said phase adjustingmeans to alter the phase of the output thereof to bear a predeterminedphasic relationship to the input to said lter, means to recombine saidadjusted carrier with the original radio signal, single sidebandselecting means, means for recombining the adjusted carrier and theselected sideband, and detector means energized from the input to saidfilter and the output therefrom to generate a direct current Voltagecorresponding to the phase relationship thereof, said voltage indicatingthe intelligence imposed on said radio signal as a func tion of thefrequency thereof.

10. ln apparatus for receiving radio signals having a carrier andassociated upper and lower sidcbands` for conveying intelligence as afunction of both the amplitude and frequency of such signal, narrow bandlter means energized by said radio signal to separate the carriertherefrom. dynamic phase adjusting means for determining the phase ofthe separated carrier, means for comparing the phasic relationship ofsaid carrier at the input to said lter and at the output of said phaseadjusting means to generate a control signal corresponding to saidrelationship, said control signal being applied to said phase adjustingmeans to alter the phase of the output thereof to bear a predeterminedphasic relationship to the input to said lter, means to recombine saidadjusted carrier with the original radio signals, upper sidebandselecting means, means for recombining the adjusted carrier signal andthe selected upper sideband, lower sideband selecting means, means forcombining the adjusted carrier signal and the selected lower sidehand.and detector means energized from the input to said lter and the outputtherefrom to generate a direct current voltage corresponding to thephase relationship, said voltage indicating the intelligence imposed onsaid radio signal as a function of the frequency thereof.

l1. ln apparatus for operating on modulated periodic voltage signalscomprising a carrier and associated side bands, sensing means comprisinga narrow band, filter energized from said signals to isolate saidcarrier, and two balanced demodulators each energized with said signalsand said isolated carrier and having means for generating a directcurrent voltage having a magnitude and polarity corresponding to thephase of the periodic voltage relative to said carrier, the outputs ofsaid balanced demodulators being serially connected to produce equal andopposite direct current voltage outputs for a given change in themagnitude and frequency of such periodic voltage signal.

l2. In apparatus for operating on modulated periodic voltage signalscomprising a carrier and associated side bands, sensing means comprisinga narrow band filter energized from said signals to isolate saidcarrier, two balanced dcmodulators each energized with said signals andsaid isolated carrier and having means for generating a direct currentvoltage having a magnitude and polarity corresponding to the phase ofthe periodic voltage relative to said carrier, the outputs of saidbalanced demodulators being serially connected to produce equal andopposite direct current voltage outputs for a given change in magnitudeand frequency of such periodic voltage signal, and means energized fromthe output of said balanced demodulators to control the phase of theoutput of said narrow band filter.

References Cited in the tile of this patent UNITED STATES PATENTS2,041,855 Ohl May 26, 1936 2,193,801 Byrne Mar. 19, 1940 2,266,517 Rustet al Dec. 16, 1941 2,494,323 Weber Ian. 10, 1950 2,512,530 OBrien etal. June 20, 1950 2,516,009 Mack et al. July 18, 1950 2,551,348 SunsteinMay 1, 1951 2,575,047 Crosby Nov. 13, 1951 2,611,825 Harris Sept. 23,1952 2,619,547 Ross Nov. 25, 1952 2,623,169 Gardere Dee. 23, 1952

